This invention relates to an improved fuel assembly for a boiling water reactor with an increased burnup.
A boiling water reactor, which will be hereinafter referred to as "BWR", has an axial void distribution, and thus neutron moderation is faster and the reactivity is higher in the lower region of a core than in the upper region, and the power distribution is so deformed that the axial power peak position is shifted downwards in the core.
According to the conventional core design, a linear heat generation ratio (power per unit length of fuel rod) is designed to be kept low by controlling the axial power peak as much as possible in view of maintenance of fuel soundness and increase in capacity factor, and thus it has been proposed that control rods are inserted shallowly at the axial power peak position in the lower region of a core (these control rods are called "shallow control rods") or fuel rods containing gadolinia (Gd.sub.2 O.sub.3) as a burnable poison for controlling the reactivity are inserted at the axial power peak position (U.S. Pat. No. 4,229,258).
Axially-zoned fuels having different enrichments in the upper and lower regions of the fuel assembly recently developed have a lower uranium enrichment in the lower region of the fuel assembly than in the upper region thereof, where the infinite neutron multiplication factor is decreased in the lower region to reduce the power peak and flatten the power distribution. The axially-zoned fuels have been found to have a particularly distinguished effect on flattening of axial power distribution, as compared with use of said shallow control rods and use of fuel rods containing burnable poison at the axial power peak position and have been increasingly used (said U.S. Pat. No. 4,229,258).
However, as a result of recent development of fuel technology, PCI (pellet cladding interaction) resistant fuels such as a barrier fuel using a cladding with a Cu or Zr-plated inner surface have been proposed [Japanese Patent Application Kokai (Laid-open) No. 51-69792 corresponding to U.S. Patent Application Ser. No. 522,767]. As a result, the flattenting of power distribution so far utilized has not been particularly required, and the linear heat generation ratio can be increased so far as the soundness of fuel can be maintained, that is, without any fear of damage at cladding. In such a core, a new core design utilizing the feature of BWR has been in demand without any consideration of flattening of power distribution. That is, BWR has such a feature that in a fuel used in a high void fraction a neutron spectrum is more hardened than in a fuel used in a low void fraction, and consequently plutonium accumulation tends to increase and this tendency is more pronounced with burning of fuel. Thus, an operation to increase burnup by conducting operation in a high void fraction in an operating cycle to increase formation of plutonium and reducing the void fraction at the end of cycle to shift the neutron spectrum and increase the reactivity of plutonium so far formed has been proposed and called "spectral shift operation" (Churlik, D. G. et al: Extending BWR Burnup with Special Shift Control, papers disclosed at American Nuclear Society Topical Meeting on LWR Extended Burnup-Fuel Performance and Utilization, Apr. 4-8, 1982, Volume 2, pages 7-91 to 7-107).
The reactivity gain at the end of cycle by the spectral shift operation can be obtained by reduction in average void fraction in a core and upward shifting of axial power distribution.
Owing to a feature of BWR that the power distribution is shifted downwards in the beginning of cycle, BWR is advantageous for the spectral shift effect. To utilize the spectral shift to a maximum in BWR, it is necessary to give a BWR such a characteristic as a larger power in the lower region of a core and a higher void fraction in the beginning of cycle and a larger power in the upper region and a lower void fraction at the end of cycle.
A step for satisfying this necessity is to use a fuel assembly having a lower total gadolinia content in the upper region of the fuel assembly than in the lower region thereof and an equal fuel enrichment both in the upper and lower regions to increase the reactivity in the upper region of the core at the end of cycle. However, such a problem as a decrease in reactor shutdown margin, i.e. a safety margin for keeping a reactor in a non-critical state by inserting control rods even in a cold state of reactor remains to be solved even in that case. That is, a power distribution of reactor in a cold state has a peak in the upper region of the core, and if the total gadolinia content in the upper region of the fuel assembly is low, the control capacity of gadolinia is lowered, and the reactor shutdown margin is decreased.